The present invention relates to a spectrometric instrument comprising a scanning interferometer and more particularly comprising a scanning interferometer operating according to the Michelson principle or a principle derived there from (generally referred to in this specification as a “Michelson type” interferometer).
Known scanning interferometers, such as those of the Michelson type, generally comprise a beamsplitter (typically also including a compensator) and two or more reflectors, such as mirrors or retro-reflectors, with at least one of the reflectors being arranged to be reciprocally translatable. Collimating lenses or other optics may also be associated with the interferometer but are not fundamental to its operating principle which relies essentially on the presence of a beamsplitter and relatively movable reflectors.
It is understood that a scanning interferometer refers to an optical arrangement in which a beam is first split by a beamsplitter into two components which are subsequently recombined to interfere with one another after each having traversed a different path that is delimited by a respective one of a pair of relatively moveable reflectors. Information may then be derived from the spectral contents of the interference which relates to a property of a sample with which the beam has interacted.
When such an interferometer is, for example, employed in a spectrometric instrument for optical spectroscopy, an observation beam consisting of relatively broad band radiation in a wavelength region of interest is launched into the interferometer to strike the beamsplitter. In this context the term “launched” refers to a beam being transmitted from a last optical element, such as a light source, a fiber optic end, a lens or other optical element which may affect the beam path or shape. This observation beam is split into essentially two parts of equal intensity at the beamsplitter. A first beam is reflected by the beamsplitter and travels along a first ‘arm’ of the interferometer to the first reflector from where it is reflected back to the beamsplitter. A second beam is transmitted through the beamsplitter and travels along a second ‘arm’ to the second reflector from where it is also reflected back to the beamsplitter to overlap the reflected first beam. The retardation, δ, is the difference between the optical path lengths of the two arms and depending on the retardation each wavelength of the spectral source may interfere destructively or constructively when the back-reflected light in the two arms overlap on the beamsplitter. The intensity pattern of the overlapping, interfering light as a function of retardation is known as an interferogram. The interferogram is recorded by a detector as the one or more reflectors are moved to create cyclic excursions of the related optical path and hence a cyclic optical path length difference between the first and the second beams. As a result of this each wavelength in the observation beam is modulated at a different frequency. Spectral information may then be extracted from this observation interferogram by numerically performing a Fourier transform (FT).
When recording an observation interferogram, particularly when using the so-called Fast FT technique, the sampling of the output of the associated detector at exact equidistant positions of the translatable reflector is critical for avoiding error.
It has become a well established practice in FT spectroscopy to use a monochromatic source of radiation of known wavelength, A, such as a laser, to generate a reference beam. This reference beam is employed in the scanning interferometer to determine the required exact equidistant positions and one such FT interferometer is disclosed in U.S. Pat. No. 6,654,125. Here, as is common, the reference beam is launched into the scanning interferometer simultaneously with the observation beam and is made to follow a light path through the optical components of the interferometer that is substantially parallel to that followed by the observation beam. As with the observation beam the reference beam is split into two beams of substantially equal intensity by the beamsplitter. A reference interferogram is generated by the two back-reflected portions of reference beam upon their overlap at the beamsplitter to be detected by an associated detector. This reference interferogram is sinusoidal having a period of oscillation on the retardation axis δper, that is directly related to the wavelength as: δper=λ/2 (1)
Since the wavelength of the reference beam is accurately known then periodically occurring features, such as zero crossing positions, of the reference interferogram can be employed to accurately determine the incremental displacement and/or velocity of the translatable reflector in the interferometer. Thus the sampling time for the observation interferogram may be accurately determined.
A problem associated with the known scanning interferometer design is that the launch of the reference beam into the interferometer either requires additional optical components or obstructs the observation beam path. The reference beam may, for example, be launched by using periscope mirrors or through a hole in any collimating optics for the observation beam. In both cases however, a part of the observation beam is blocked. Alternatively, the reference beam may be launched into the interferometer using a dichroic mirror but this also gives rise to a reduction in the total power of the observation beam through the interferometer and also requires space in the observation beam path.